1. Field of the Invention
This invention relates to inkjet printing apparatus and methods for inkjet printing using ink that is curable upon exposure to actinic radiation such as UV radiation. More particularly, the present invention is directed to automated methods and apparatus for optimizing the quality of an image obtained by using an inkjet printer and radiation curable ink.
2. Description of the Related Art
Inkjet printing has increased in popularity in recent years due to its relatively high speed and excellent image resolution. Moreover, inkjet printing apparatus used in conjunction with a computer provides great flexibility in design and layout of the final image. The increased popularity of inkjet printing and the efficiencies in use have made inkjet printing an affordable alternative to previously known methods of printing.
In general, there are three types of inkjet printers in widespread use: the flat bed printer, the roll-to-roll printer and the drum printer. In the flat bed printer, the medium or substrate to receive the printed image rests on a horizontally extending flat table or bed. An inkjet print head is mounted on a movable carriage or other type of mechanism that enables the print head to be moved along two mutually perpendicular paths across the bed. The print head is connected to a computer that is programmed to energize certain nozzles of the print head as the print head traverses across the substrate, optionally using inks of different colors. The ink on the substrate is then cured as needed to provide the desired final image.
In roll-to-roll inkjet printers, the substrate to receive the printed image is commonly provided in the form of an elongated web or sheet and advances from a supply roll to a take-up roll. At a location between the supply roll and the take-up roll, a print head is mounted on a carriage that is movable to shift the print head across the substrate in a direction perpendicular to the direction of advancement of the substrate. Known roll-to-roll inkjet printers include vertical printers, wherein the substrate moves in an upwardly direction past the print head, as well as horizontal printers, wherein the substrate moves in a horizontal direction past the print head.
Drum inkjet printers typically include a cylindrical drum that is mounted for rotational movement about a horizontal axis. The substrate is placed over the periphery of the drum and an inkjet print head is operable to direct drops of ink toward the substrate on the drum. In some instances, the print head is stationary and extends along substantially the entire length of the drum in a horizontal direction. In other instances, the length of the print head is somewhat shorter than the length of the drum and is mounted on a carriage for movement in a horizontal direction across the substrate.
Inks that are commonly used in inkjet printers include water-based inks, solvent-based inks and radiation-curable inks. Water-based inks are used with porous substrates or substrates that have a special receptor coating to absorb the water. In general, water-based inks are not satisfactory when used for printing on non-coated, non-porous films.
Solvent-based inks used in inkjet printers are suitable for printing on non-porous films and overcome the problem noted above relating to water-based ink. Unfortunately, many solvent-based inks contain about 90 percent organic solvents by weight. As solvent-based inks dry, the solvent evaporates and may present an environmental hazard. Although environmental systems may be available for reducing the emission of solvents to the atmosphere, such systems are generally considered expensive, especially for the owner of a small print shop.
Furthermore, inkjet printers using either solvent-based inks or water-based inks must dry relatively large quantities of solvent or water before the process is considered complete and the resulting printed product can be conveniently handled. The step of drying the solvents or water by evaporation is relatively time-consuming and can be a rate limiting step for the entire printing process.
In view of the problems noted above, radiation-curable inks have become widely considered in recent years as the ink of choice for printing on a wide variety of non-coated, non-porous substrates. The use of radiation curing enables the ink to quickly cure (commonly considered as xe2x80x9cinstantxe2x80x9d drying) without the need to drive off large quantities of water or solvent. As a result, radiation curable inks can be used in high speed inkjet printers that can achieve production speeds of over 1000 ft2/hr (93 m2/hr.)
Inkjet printers that are capable of printing on relatively large substrates are considered expensive. Accordingly, it is desired to use the same printer to impart images to a wide variety of substrates using a wide variety of ink compositions if at all possible. Moreover, it is preferred that each image printed by such printers be of high quality on a consistent basis regardless of the type of substrate and the type of ink used, in view of the time and expense of reprinting the image in instances where the quality of the image is less than desired.
From a practical standpoint, many print shops use the same type of ink on a variety of different substrates, since a considerable amount of time might otherwise be needed to change from one type of ink to another. However, ink of a given formulation may interact differently with different types of substrates. It is possible that the quality of the final printed image may be significantly impaired when the composition of the substrate is changed from one type to another.
The printer operator is often provided with little guidance as to the selection of process parameters that will provide the best image quality for any combination of ink and substrate. Today, many operators use a manual trial-and-error methodology in an attempt to optimize the parameters of the printing process. For example, the operator may print a number of images and vary the curing time or temperature of the curing device. Once the images have cured, the operator visually reviews each image for image quality in order to help select an optimal temperature and/or curing time.
Skilled operators of inkjet printers tend to accumulate over time a knowledge of preferred printing parameters for use when certain combinations of ink and substrate are selected. Unfortunately, this wealth of knowledge cannot be readily passed along to newer operators who are relatively unskilled in the art of inkjet printing. Consequently, it would be desirable to provide an automated method and apparatus for inkjet printers that would consistently enable high quality images to be printed without undue reliance upon the operator""s skill and level of experience.
The present invention is directed toward automated methods and apparatus for selecting process parameters used in inkjet printing with radiation curable ink such as ultraviolet xe2x80x9cUVxe2x80x9d radiation curable ink. Preferred dwell times for certain combinations of selected inks and selected substrates are stored in computer memory and recalled as needed. A control device varies the dwell time in order to provide the desired result.
In more detail, the present invention is directed in one aspect to inkjet printing apparatus for curable ink that comprises a support for receiving a substrate and a print head for directing radiation curable ink toward a substrate received on the support. The apparatus also includes a curing device for directing radiation toward ink received on the substrate, and a controller having an input for receiving one or more characteristics of the substrate and one or more characteristics of the ink. The controller includes a computer for determining a desired dwell time for the ink based on the characteristics of the substrate and the ink. The apparatus further includes a control device connected to the computer for varying the dwell time in accordance with the desired dwell time determined by the computer.
The present invention is directed in another aspect toward a method of inkjet printing. The method includes the acts of selecting a radiation curable ink and selecting a substrate. The method also includes the act of entering at least one characteristic of the ink and at least one characteristic of the substrate into a computer. The method further includes the act of determining a preferred ink dot gain when the selected ink is printed onto the selected substrate, and calculating with the computer a dwell time for achieving the preferred ink dot gain.